you could try too make gun jamming and a way to fix it like maybe take 2 seconds of holding reload key it could add a lot to the mechanics and make gun fights a lot more intense and if your planning on making it coop you should add a way to interact with your teammate and heal them and make it so when healing your self you can reach all of the wounds so you don't get full health without a teammate though these could be hard to implement so you can always hold out until later in the development of the game